This invention relates to non-invasive, small-perturbation measurements of macroscopic mechanical properties of organs and blood vessels to evaluate tissue pathology and body function.
Pathological tissue changes are often correlated with changes in density, elasticity and damping. While microscopic mechanical changes are sometimes correlated with ultrasound transmission and reflection properties, many important mechanical changes are manifested most clearly on a large scale at low frequencies. For these, manual palpation remains almost the sole diagnostic tool. Considerable effort has gone into blood pressure measurement methods, but not by analyzing small-perturbation mechanical properties of the pressurized vessel. Intraocular pressure is sensed by causing small eyeball shape perturbations, either by flattening a predetermined area of the sclera against an instrument surface, or by distorting the eyeball with a calibrated puff of air and measuring the deflection of a light beam reflected off the eye. The current invention similarly induces small shape perturbations, but obtains better data with greater patient comfort through a sophisticated use of vibrational excitation and mechanical response measurement and analysis.
Blood pressure measurement methods are commonly invasive or cause temporary occlusion of blood flow. A common measurement involves a catheter inserted into the radial artery and advanced to the aorta. Pulmonary arterial and capillary pressures are measured by the Swan Ganz method, whereby a catheter is advanced via a large vein and through heart valves into the right atrium, through the right ventricle, into the pulmonary trunk and up to the T-branching of right and left pulmonary arteries. Pulmonary arterial pressure is measured through the catheter, while pulmonary capillary pressure is obtained by inflating a balloon at the catheter tip to occlude flow to one lung while measuring the fall in pressure distal to the balloon, approaching capillary pressure. The trauma and risk of these invasive methods is apparent.
Non-invasive occlusive blood pressure methods commonly employ a pressurized cuff surrounding an arm or leg to collapse an underlying artery. The moments of collapse and reinflation, marking the times when blood pressure drops below and rises above cuff pressure, are sensed from blood flow noise (by stethoscope or contact microphone), by ultrasound doppler flow detection or from a sudden change in limb cross-section (sensed by monitoring of pressure or volume in the occluding cuff or a sensing cuff placed distal to the occluding cuff). The result is usually an estimate of systolic and diastolic extremes of pressure. Where cuff pressure pulsations are sensed as average cuff pressure is varied, mean arterial pressure can be estimated. These cuff methods depend on a steady heartbeat and cannot follow irregular beat-to-beat pressure fluctuations.
Recent servo cuff methods overcoming some of the above difficulties include those described by Aaslid and Brubakk, Circulation, Vol. 4, No. 4 (ultrasound doppler monitors brachial artery flow while a servoed cuff maintains fixed, reduced flow) and Yamakoshi et al, "Indirect Measurement of Instantaneous Arterial Blood Pressure in the Human Finger by the Vascular Unloading Technique", IEEE Trans. on Biomedical Eng., Vol. BME-27, No. 3, March 1980 (a similar system optically monitors capillary blood volume in the finger while a servoed cuff maintains a constant optical reading). The former method yields a continuous pressure reading but blocks venous return flow so that monitoring must be interrupted frequently. The finger pressure waveform of the latter method is distorted relative to the important pressure waveform loading the heart and central arteries.
D. K. Shelton and R. M. Olson, "A Nondestructive Technique To Measure Pulmonary Artery Diameter And Its Pulsatile Variations", J. Appl. Physiol., Vol. 33, No. 4, Oct. 1972, used an ultrasound transducer in the esophagus to track canine pulmonary artery diameter. They reported approximate short-term pressure/diameter correlation, but Itzchak et al, "Relationship of Pressure and Flow to Arterial Diameter", Investigative Radiology, May-June, 1982, using ultrasound to track canine systemic arterial diameter, found no useful longterm pressure/diameter correlation. Hence, it is doubtful that blood pressure can be calibrated against measured vessel diameter for purposes of continuous monitoring.
In other areas of the human body, Kahn, U.S. Pat. No. 3,598,111, describes a mechanically and acoustically tuned pneumatic system, useful at a single frequency, for measuring the impedance of the air passages and tissues of human lungs to obtain a two-component trace (representing resistive and reactive impedance) as a function of time.